This invention relates to paper coating compositions based on alkali-swellable latices.
Alkali swellable latices are those latices which are pH-sensitive in that they provide paper coating compositions which display increasing viscosities with increasing pH in the alkaline range of pH values. These latices are also characterized by an upward drift in viscosity on standing after addition of alkali. This pH-sensitivity is thought to result from the presence in the polymer molecules of groups such as carboxylic and ester groups which are capable of forming inter-molecular hydrogen bonds as favourable steric conditions arise during the Brownian motion of the molecules. As the hydrogen bonding progresses, the viscosity of the system rises and eventually a gel is formed if the hydrogen bonding is allowed to continue long enough.
Use of alkali-swellable latices as binders in paper coating compositions is desirable since the swelling action induced by the addition of alkali involves the absorption of water and thus improves the water retention and reheological properties of the coating composition. The drying properties are also improved since the absorbed water tends to be retained in the coating and not soaked up immediately by the paper.
While alkali-swellable characteristics are desirable in paper coating compositions, it is necessary for optimum paper coating operations that the viscosity rise be stopped and levelled off at the desired point. This increase in viscosity can be controlled to some extent by supplying kinetic energy to the system such as by agitation, and also by heating, by limiting the amount of near-the-surface carboxylation in the polymer particles and by being careful to avoid the addition of excess alkali to the system. For commercial scale operations it is desirable to have a means for controlling the viscosity of an alkali-swellable coating composition at the optimum level which is more reliable and requires less care and allows for more leeway for variation in operations than is presently available.